MKTTIohS I'oR TKSTINC, 11 A KDN KSS.
ness of the material. An apparatus was also used having an obtuse-angled hardened point which would fall from a height upon the specimen to be tested, and according to the size of the indentation made the hardness was defined. A late 'method is that of testing-hardness by means of electricity, in which a current passes through the specimen to be tested and through other standard pieces. The current necessary to produce fusion is observed and compared with that of the normal pieces when they fuse.
Up to about 1900 the beat device we had for testing relative decrees in the hardness of metals is that of Professor Thomas Turner, who stood at the head of professional men in advancing knowledge on iron, etc. It affords the author much pleasure to here present a cut of the device, accompanied by a description in the professor's own lan^'tia^e:
My first arrangement is as follows, Ki#. <)i: It consists of a balanced and j^raduutcd bruin of i;-un metal A working on steel knife ed;»j;es B and counterposed by means of a lan;"e slidiny; weight F, tlie final adjustment, bein.n1 obtained by the screw t). Wlien balanced, it is sensitive to o.oi gramme at 1C, though such delicaey is not probably required. Tin- knife edv;vs rest upon plain-s in tin* support (', whirls is eapahle of rotating on a steel pivot eon-iieeted with the rod I). The diamond is mounted in a brass tube, havin;..1,1 a milled head which is fixed by means of a screw at 1C. The specimen to be tested, which often takes the form shown, J, s-. supported by a wooden block K. The weight II is arranged r.o that each division on the j^radualetl scale shall eonvspoud to a pressure dj a gramme at the diamond point. Thus, at division r,», we have a pressure of 12 grammes on the diamond. Three estra weights, I, are used when necessary. Tlniy are each of the same weight as II. Uc-nee, with one weight, scale division m eorre:,ponds to m y^rammes on the diamond, with two weights i«> corresponds to '/o grammes,with thr<*e weights to -\n ^nimmes, and \«-ith tour weights to ,|o grammes, the other scale division:, in Volume XXIII. of the .*